Yesterday Dell announced its quad 7800GTX SLI motherboard, but is such a monstrosity something to really value, or is it the ultimate White Elephant?

When Michael Dell and Jen-Hsun Huang stood up in front of CES together
and announced the Dell XPS 600 Renegade, a product manager for a major
motherboard manufacturer began to laugh hysterically. Dual
7800GTX video cards are nothing new -- I was shown several prototypes
back in October. Putting two of those cards in an SLI
configuration is something that has been feasible for several months,
but only on beta or severely hacked drivers. NVIDIA's partnership
with Dell means they will now need to provide quad SLI support inside
the core Detonator drivers, or release specific drivers for these
systems. The first option leaves the possibility for some
interesting driver hacks, the second option might not be practical
depending on the actual volume of these systems.

But pragmatically looking at the performance of such a system, what can
we really expect? Many modern games only receive a 40%
performance boost by putting 7800GT cards into dual SLI mode
at modest resolutions, like 2048x1536. Dual 7800GTX cards receive
only about a 33% performance boost in the same benchmarks, but out of
the gate a single 7800GTX out performs two 7800GT cards in SLI.

Diminishing returns is already apparent with only two high
performance 7800GTX cards, and there are similar results elsewhere for
7800GTX 512MB cards. Although there is no claim anywhere that
this new XPS Renegade will be a mainstream system, hopefully we see
more innovative and efficient (maybe even quiet) systems from the
world's largest PC manufacturer, rather than another dirty hack to grab
headlines... Fortunately Dell just released their 3007FPW to calm our tensions.

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Diminishing returns implies that you lose efficiency. I'm not saying that going from 2 card SLI to 4 card SLI will give you the same return or that the efficiency will be the same, but rather that using values under which they are clearly CPU limited and explaining that as diminishing returns is not correct. In truth the performance benefit potential from going SLI is somewhere between 80-90% (with 2 cards, dunno about 4) but with the most powerful cards your CPU becomes a huge bottleneck (heck, an FX57 may even bottleneck a single GTX7800 512 in some programs). Sure there is no CPU to match the rendering power provided by these setups but claiming that these dont perform much better than 1 card systems is due to diminishing returns is akin throwing a low FSB on a high clock P4 and then claiming that performance bites because clockspeed doesn't help much at all.

The law of diminishing returns holds that, holding everything else constant, adding more gives you less. This applies, for example, with eating cheeseburgers. You get quite a bit of utility from one cheeseburger, and perhaps nearly as much from a second, but almost nothing from a third, and possibly even negative return on a fourth (upset stomach or vomiting, for example).

So in this case adding on 7800GTX is awesome because it outperforms SLI 7800GT, which itself is 40% better than a single 7800GT; the point of diminishing returns hits early though, because adding a second 7800GTX in the form of a Dual 7800GTX only gives you a 33% improvement over the 7800GTX, and therefore in theory adding two more 7800GTXs, in the form of a Dual 7800GTX will probably add little exactly because they are CPU, memory, or bus limited, which is the very meaning of the term "diminishing returns".

If you can rejigger the busses, or the CPU, or the ram so that adding a Dual 7800GTX improves performance another 45%, then you have broken out of "diminishing returns" and into the land of "significant improvement". But since we aren't changing the CPU, bus, or ram when we add another Dual 7800GTX, we live squarely in the land of diminishing returns.